Random access channel priorization scheme

ABSTRACT

The present invention relates to a method and to devices for transmitting and receiving data in a code division multiple access telecommunication system. A random access time window ( 20 ) is provided, which comprises a plurality of random access slots for transmitting random access data, for example from a mobile station ( 24 ) to a base station ( 25 ). The plurality of random access slots in the random access time window ( 20 ) is divided into at least two groups, whereby the groups are allocated to respective priority classes, whereby the priority classes represent the transmission priorities of the random access data to be transmitted in the random access slots. The present invention allows a reduction in congestion on the random access slots for certain access types at the expense of other access types and a more efficient use of the random access time window.

The present invention relates to a method and to devices fortransmitting and receiving data in a code division multiple accesstelecommunication system.

A telecommunication system is a system, in which data are communicatedbetween one or more base stations or one or more mobile stations.Thereby, the communication area is divided in cells, in which one basestation communicates with one or more mobile stations. Multiple accessstations are used to support the simultaneous access of a plurality ofmobile stations to one base station within delimited resources of thetransmission system. Several multiple access systems are known, forexample frequency division multiple access (FDMA), time divisionmultiple access (TDMA), or code division multiple access (CDMA).Additional to these basic types of multiple access systems, combinationsof these different systems are possible and in practical use. TheGSM-system for example uses a combination of FDMA and TDMA.

The present invention particularly relates to the transmission andreception of random access data in a code division multiple accesssystem. Random access data are transmitted in the so-called randomaccess channel (RACH) from a mobile station to build up a connectionwith a base station. The random access data from the mobile station cancontain a request, if the base station has sufficient resourcesavailable to build up the required connection.

The random access channel comprises or consists of succeedingly orperiodically provided random access time windows, in which severalrandom access slots are available. An example for such a known randomaccess time window is shown in FIG. 1. The different random access slotsare randomly chosen by a mobile station for the transmission of randomaccess data. In a currently proposed wide band direct sequence CDMA(WCDMA) system the random access time windows are based upon an initialpreamble scrambling code, which differentiates between one cell andanother cell. Thereby, these codes need to be planned to ensure thatneighbouring cells do not use the same code. Therefore, within thepreamble part of each random access burst, is provided the preamblesignature, which is one of 16 separate codes available for use withinthat cell. These 16 codes can be seen as separate slots. One of thesecodes is chosen randomly by the mobile station for the transmission ofrandom access data. Beforehand, the base station broadcasts, which codesare available in each cell over the broadcast control channel (BCCH). Inaddition, within one time frame (10 ms) are provided 8 time offsets,each of 1,25 ms, allowing further 8 variations. In other words, in eachtime frame a random access time window is provided, which comprises aplurality of random access slots for transmitting random access datafrom one or more mobile stations to the base station. The random accesstime window thereby extends over the time frame of 10 ms so that 128different random access slots (16 separate preamble codes and 8 timeoffsets) are provided within one random access time window. This case isshown in FIG. 1.

A collision, i.e. a situation, where the messages collide and are lost,only occurs, when both, the preamble code and the time offset are chosenin the same random access time window. In practice it is expected thatonly about 30% of the theoretical maximum of 128 accesses per 10 ms willbe possible.

In a situation, where a number of packet data users are operating in aburst traffic situation, this maximum could be quickly reached. In sucha situation the access to the network will either become slower or notbe possible at all. This is due to congestion caused by the build-up offirst time requests and the retransmissions made necessary by previouscollisions. Since the access to the random access slots is contentionbased, a guaranteed upper limit on the amount of time needed to accessthe system even after an initial bust is not ensured. For packet dataapplications, which demand a constant periodic delivery of data, readyaccess is critical.

The object of the present invention is thus to provide a method anddevices for transmitting and receiving data in a code division multipleaccess telecommunication system, in which a random access time windowcomprising a plurality of random access slots for transmitting randomaccess data is provided and which enable a reduced congestion on therandom access slots and a more efficient use of the random access timewindow.

This object is achieved by a method for transmitting and receiving datain a code division multiple access telecommunication system, comprisingthe steps of providing a random access time window comprising aplurality of random access slots for transmitting random access datafrom at least one first communication device to a second communicationdevice, dividing a plurality of random access slots of the random accesstime window into at least two groups, and allocating the groups torespective priority classes, whereby the priority classes represent thetransmission priorities of the random access data to be transmitted inthe random access slots.

The first communication device can thereby be a mobile station and thesecond communication device can thereby be a base station of thetelecommunication system.

The above object is further achieved by a device for transmitting andreceiving data in a code division multiple access telecommunicationsystem, in which a random access time window comprising a plurality ofrandom access slots for transmitting random access data is provided, theplurality of random access slots of the random access time window beingdivided into at least two groups and the groups being allocated torespective priority classes, whereby the priority classes represent thetransmission priorities of the random access data to be transmitted inthe random access slots, with means for randomly choosing one or morerandom access slots from a group having a certain priority classcorresponding to the transmission priority of the random access data tobe transmitted, and means for transmitting the random access data insaid chosen random access slot(s). This device for transmitting andreceiving data according to the present invention can for example be amobile station.

The above object is further achieved by a device for transmitting andreceiving data in a code division multiple access communication system,in which a random access time window comprising a plurality of randomaccess slots for transmitting random access data is provided, with meansfor dividing the plurality of random access slots of the random accesstime window into at least two groups, whereby the groups are allocatedto respective priority classes, the priority classes representing thetransmission priorities of the random access data to be transmitted inthe random access slots, and means for transmitting information definingthe groups of the random access time window. This device fortransmitting and receiving data according to the present invention canfor example be a base station.

According to the method and the devices of the present invention, theallocated random access channel resources are divided between differenttypes of access on a priority basis. Thus higher priority access typesreceive more physical resources. This ensures, that some access typeswill have a better possibility of gaining access to the network due tothe higher probability that a particular message will be successful inreaching the base station. The priority types and ratings can betailored by the network and broadcast to a mobile station upon thebroadcast control channel.

The method and the devices according to the present inventionparticularly lessens the effect of possible congestion on the randomaccess channel caused by increased load, for instance packet datacapacity requests, and the transfer of small amounts of user data withinrandom access bursts.

Advantageously, the transmission priorities of the random access data tobe transmitted are determined on the basis of the content and the typeof the random access data. Further, in said device for transmitting andreceiving data, which can be a base station, said means for dividing therandom access slots into groups sets the number of random access slotsin each group variably depending on system requirements. The size ofeach group is thus controlled by the network and can be dynamicallychanged depending on current traffic patterns, network requirements andso forth. A corresponding signal indicating the new groups will then betransmitted over a broadcast channel from the base station to the mobilestations.

Said device for transmitting and receiving data according to the presentinvention, which can be a mobile station, can further comprise means forextracting information defining the groups of the random access timewindow from a received broadcast signal. Each time the defined groupsare changed by the network, a corresponding signal is broadcast from therespective base station. This signal is received by the mobile stations,in which above-mentioned means for extracting the information definingthe groups extracts said information and uses the information for thefollowing transmissions of random access bursts. The access probabilitycan depend on the number of random access slots in the respectivegroups. Thus, groups with higher priority classes can comprise a largernumber of random access slots than groups with lower priority classes.

Further advantageously, each random access slot in said random accesstime window is defined by a time offset value and a preamble code.

The advantages of the method and the devices of the present inventionare a high flexibility in allowing prioritization of access related tothe access type and subsequent prevention of congestion for certainaccess types which may be considered to have a higher access priority atthe expense of those access types that may be considered to have a lowaccess priority. A further advantage of the present invention is theability to dynamically chose the quality of service level on a regularbasis depending upon traffic conditions via a broadcast channel fordifferent access types. The present invention further avoids deadlocksituations, where random access channel re-transmissions combine withnormal traffic to cause a system overload and then a collapse forcertain types of traffic at the expense of others.

A preferred embodiment of the present invention is now explained indetail relating to the enclosed drawings, in which

FIG. 1 shows a schematic diagram of a known random access time window,

FIG. 2 shows a schematic diagram of a random access time window dividedinto groups according to the present invention,

FIG. 3 shows a block diagram of a mobile station according to thepresent invention, and

FIG. 4 shows a block diagram of a base station according to the presentinvention.

In FIG. 2, a random access time window 20 is shown, in which a pluralityof random access time slots are provided. In the exemplary random accesstime window 20 shown in FIG. 2, 128 random access time slots areprovided, each of which is defined by one of 8 time of said values andof 16 preamble codes.

The random access time window shown as an example in FIG. 2 extends thusover one time frame of 10 ms, in which 8 time offsets, each of 1,25 msare provided. As shown in FIG. 2, the random access time window isdivided into three different groups of random access time slots. Each ofthe three groups has a different access priority. In the random accesstime window 20 shown in FIG. 2, 32 random access time slots 21 areallocated to initial access attempts. Initial access attempts areattempts from a mobile station to build up a connection with a basestation. This first random access slot group is defined by the firsteight preamble codes and the first four time offset values. The secondgroup of random access slots 22 comprises also 32 slots and is allocatedfor packet data access attempts. These are attempts from mobilestations, which already have build up a connection with a correspondingbase station and which to transfer packet data. The random access slots22 of the second group are defined by the first eight preamble codes andthe second four time offset values.

The third group of random access slots 23 is the largest group and isdefined by the second eight preamble codes and all possible 8 timeoffset values. The random access slots 23 of the third group areallocated for voice call access attempts. These are attempts of a mobilestation, which already has build up a connection with a base station andwishes to initiate a real time speech connection. Since the third grouphas the highest priority class, the third group has the largest numberof random access slots 23 available so that a high probability of accessis given for voice call access attempts.

In FIG. 3, a block diagram of a mobile station 24 according to thepresent invention is shown, and in FIG. 4, a block diagram of a basestation 25 according to the present invention is shown. In the blockdiagrams shown in FIGS. 3 and 4, only the elements necessary forexplaining and incorporating the present invention are represented. Itis to be understood, that both, the mobile station 24 shown in FIG. 3 aswell as the base station 25 shown in FIG. 4 comprise all other necessaryelements for transmitting and receiving data in a code division multipleaccess telecommunication system according to the present invention, forexample, coding and decoding means, interleaving and deinterleavingmeans, modulating and demodulating means and so on.

The mobile station 24 shown in FIG. 3 comprises an antenna 26, areceiving means 28 and a transmitting means 29 for receiving andtransmitting data over the antenna 26 to and from a corresponding basestation, for example, the base station 25 shown in FIG. 4. Thetransmitting means 29 is coupled to a means 30 for randomly choosing oneor more random access slots from the group having a priority class,which corresponds to the random access data to be transmitted. If, forexample, the random access data to be transmitted by the mobile station24 are packet data, the means 30 selects one or more random slots 27 inthe case of FIG. 2 from the corresponding group. The means 30 forrandomly choosing the random access slots from the corresponding groupis connected to a means 31 for extracting corresponding groupinformation from a broadcast signal received from a base station. A basestation, for example the base station 25 shown in FIG. 4, broadcastseach time the network changes the group allocation in the random accesstime windows, a corresponding signal defining the groups to theconnected mobile stations. In this case, the mobile station 24 receivesthe corresponding broadcast signal over the antenna 26 and the receivingmeans 28. The information defining the new groups in the random accesstime window is then extracted by the means 31 and transmit to the means30, which correspondingly chooses required random access slots from therespective groups. The means 30 for randomly choosing random accessslots from the corresponding group and the means 31 for extracting thegroup information are connected to other necessary elements of a mobilestation 24 operating in a code division multiple accesstelecommunication system as required.

The base station 25 shown in FIG. 4 comprises an antenna 27, a receivingmeans 32 and a transmitting means 33 for transmitting and receiving datato and from correspondingly connected mobile stations, for example, themobile station 24 shown in FIG. 3. The transmitting means 33 isconnected to a means 34 for dividing the random access slots of therandom access time windows into at least two groups. Thereby, the means34 receives information on the system requirements, for example currenttraffic patterns and so on, to determine the new groups or to change thesize of existing groups to adopt the telecommunication systemdynamically to changing requirements. The means 34 further generates acorresponding signal containing the information defining the new groupsof the random access time window, which is then broadcast by thetransmitting means 33 to the connected mobile stations. The receivingmeans 32 as well as the means 34 for dividing the random access timewindows into groups and generating a corresponding information signalare further connected to all the elements which are necessary to operatethe base station 25 in a code division multiple access telecommunicationsystem according to the present invention. These elements, however, arenot shown for the sake of clarity.

According to the present invention, the available random access slots inthe random access time windows 20 are divided into groups, for example,a first group of random access slots 21 allocated to initial accessattempts, a second group of random access lots 22 allocated for packetdata access attempts and a third group of random access lots 23allocated for voice call access attempts. These groups are mapped todifferent priority classes and the size, i.e. the number of randomaccess slots, of each group may depend upon current traffic levels andthe required quality of service for that priority class. It is therebypossible, that the contents of one group may overlap with another. Whena random access message is to be sent upon the random access channel,then this message will fall into one of the priority classes, which willcorrespond to one group of random access slots. Which priority class themessage falls into depends upon the message type and content. From thiscorresponding group a random choice will be made from the mobile station24 regarding the random access slot or slots to be used. This ensuresthat a message from a certain priority class has a probability ofsuccessful access to the network equal to that of other messages of thesame class sent by other mobile stations in the cell, but notnecessarily as other random access messages of different priorityclasses. The size, i.e. the number of random access slots of the groupdictates the probability of the access. The size of each group iscontrolled by the base station 25 and can be dynamically changed by themeans 34 for dividing the random access time windows into groups and bybroadcasting a corresponding information to the connected mobilestations. The means 34 thereby divides a random access time window 20into groups depending upon current traffic patterns, networkrequirements or any other predesigned control parameter.

The information signal containing information on the groups in therandom access time windows 20 which is broadcast by the transmissionmeans 33 of the mobile station 25, can be a simple pair of digits. Iffor example the random access time window 20 of FIG. 2 is taken, theaddress of the random access slots could be taken as (0,3) for the timeoffset value access and (0,7) for the preamble code access, which wouldindicate the random access time slots 21 indicated as being reserved forinitial access attempts. As stated above, the probability of access foreach group can be changed dynamically by the network. In this fashion,control is allowed over the quality of service applied to particularaccess types.

An additional feature for reducing congestion on the random access slotsand a more efficient use of the random access time window is topartition the groups of the random access time window respectively intoat least a first section and a second section. The first sectioncontains only contention based random access slots, whereas the secondsection only contains reservation based random access slots. The randomaccess slots of the first sections of the groups are used for the usualcontention based random access attempts, whereas the random access slotsin the second sections can be reserved in succeeding random access timewindows for the transfer of larger amounts of data, for example packetdata. The partitioning of each group in the random time access windowscan thereby also be dynamically controlled by the network depending onnetwork requirements. A corresponding signal is broadcast to the mobilestations to change the partitioning of the groups.

What is claimed is:
 1. Method for transmitting and receiving data in acode division multiple access telecommunication system, comprising thesteps of: providing a random access time window comprising a pluralityof random access slots for transmitting random access data from at leastone first communication device to a second communication device, anddividing the plurality of random access slots of the random access timewindow into at least two groups each having a respective initial size,and allocating the groups to respective priority classes, whereby thepriority classes represent the transmission priorities of the randomaccess data to be transmitted in the random access slots, whereby thesize of at least one of said groups is changed in accordance withchanging needs such that a probability of access for the at least onegroup is dynamically changed, and whereby contents of one of said groupsare permitted to overlap with another of said groups.
 2. Method fortransmitting and receiving data according to claim 1, characterized in,that the transmission priorities of the random access data to betransmitted are determined on the basis of the content and the type ofthe random access data.
 3. Method for transmitting and receiving dataaccording to claim 1 characterized in, that the number of random accessslots in each group is variably set depending on system requirements. 4.Method for transmitting and receiving data according to claim 1,characterized in, that a first communication device, for transmittingrandom access data of a certain transmission priority, randomly choosesone or more random access slots from the group having the correspondingpriority class.
 5. Method for transmitting and receiving data accordingto claim 4, characterized in, that the access probability depends on thenumber of random access slots in the group.
 6. Method for transmittingand receiving data according to claim 1, characterized in, that saidsecond communication device periodically broadcasts information definingthe groups of the random access time window to the at least one firstcommunication device.
 7. Method for transmitting and receiving dataaccording to claim 1, characterized in, that each random access slot insaid random access time window is defined by a time offset value and apreamble code.
 8. Device for transmitting and receiving data in a codedivision multiple access telecommunication system, in which a randomaccess time window comprising a plurality of random access slots fortransmitting random access data is provided, the plurality of randomaccess slots of the random access time window being divided into atleast two groups each having a respective initial size, and the groupsbeing allocated to respective priority classes, whereby the priorityclasses represent the transmission priorities of the random access datato be transmitted in the random access slots, with means for randomlychoosing one or more random access slots from a group having a certainpriority class corresponding to the transmission priority of the randomaccess data to be transmitted, means for transmitting the random accessdata in said chosen random access slot(s), whereby the size of at leastone of said groups is changed in accordance with changing needs suchthat a probability of access for the at least one group is dynamicallychanged, and whereby contents of one of said groups are permitted tooverlap with another of said groups.
 9. Device for transmitting andreceiving data according to claim 8, characterized in, that the accessprobability depends on the number of random access slots in the group.10. Device for transmitting and receiving data according to claim 8,characterized by, means for extracting information defining the groupsof the random access time window from a received broadcast signal. 11.Device for transmitting and receiving data according to claim 8,characterized in, that each random access slot in said random accesstime window is defined by a time offset value and a preamble code. 12.Device for transmitting and receiving data in a code division multipleaccess telecommunication system, in which a random access time windowcomprising a plurality of random access slots for transmitting randomaccess data is provided, with means for dividing the plurality of randomaccess slots of the random access time window into at least two groupseach having a respective initial size, whereby the groups are allocatedto respective priority classes, the priority classes representing thetransmission priorities of the random access data to be transmitted inthe random access slots, means for transmitting information defining thegroups of the random access time window, whereby the size of at leastone of said groups is changed in accordance with changing needs suchthat a probability of access for the at least one group is dynamicallychanged, and whereby contents of one of said groups are permitted tooverlap with another of said groups.
 13. Device for transmitting andreceiving data according to claim 12, characterized in, that thetransmission priorities of the random access data to be transmitted aredetermined on the basis of the content and the type of the random accessdata.
 14. Device for transmitting and receiving data according to claim12, characterized in, that said means for dividing the random accessslots into groups sets the number of random access slots in each groupvariably depending on system requirements.
 15. Device for transmittingand receiving data according to claim 12, characterized in, that eachrandom access slot in said random access time window is defined by atime offset value and a preamble code.
 16. Method for transmitting andreceiving data in a code division multiple access telecommunicationsystem, comprising the steps of: providing random access resourcescontaining a plurality of random access slots for transmitting randomaccess data; dividing the random access resources by assigning asignature which is available as one of 16 separate codes in accordancewith priority classes which represent transmission priorities of therandom access data to be transmitted; and randomly choosing one or morerandom access slots from the divided random access resource having thepriority class corresponding to the transmission priority of the randomaccess data to be transmitted in order to allocate the random accessdata into the one or more random access slots within the divided randomaccess resource.
 17. A device for transmitting and receiving data in acode division multiple access telecommunication system having randomaccess resources containing a plurality of random access slots fortransmitting random access data, said device comprising; means fordividing the random access resources by assigning a signature which isavailable as one of 16 separate codes in accordance with priorityclasses which represent transmission priorities of the random accessdata to be transmitted; and means for randomly choosing one or morerandom access slots from the divided random access resource having thepriority class corresponding to the transmission priority of the randomaccess data to be transmitted in order to allocate the random accessdata into the one or more random access slots within the divided randomaccess resource.
 18. Method for transmitting and receiving data in acode division multiple access (CDMA) telecommunication system having abase station and plural mobile stations, comprising the steps of;providing random access resources containing a plurality of randomaccess slots for transmitting random access data; selecting an availablesignature from 16 separate codes for differentiating one cell of theCDMA telecommunication system from another cell in accordance withpriority classes which represent the transmission priorities of therandom access data to be transmitted; and broadcasting the selectedsignature to a mobile station to enable the mobile station to randomlychoose one or more random access slots corresponding to the selectedsignature.
 19. Apparatus for transmitting and receiving data in a codedivision multiple access (CDMA) telecommunication system having randomaccess resources containing a plurality of random access slots fortransmitting random access data, said CDMA telecommunication systemhaving a base station and plural mobile stations, said apparatuscomprising: means for selecting an available signature from 16 separatecodes for differentiating one cell of the CDMA telecommunication systemfrom another cell in accordance with priority classes which representthe transmission priorities of the random access data to be transmitted;and a transmitter for broadcasting the selected signature to a mobilestation to enable the mobile station to randomly choose one or morerandom access slots corresponding to the selected signature.
 20. Methodfor transmitting and receiving data in a code division multiple accesstelecommunication system, comprising the steps of; providing randomaccess resources containing a plurality of random access slots fortransmitting random access data; dividing the random access resources byassigning a signature which is available as one of 16 separate codes onthe basis of content and type of the random access data; and randomlychoosing one or more random access slots from the divided random accessresource corresponding to the content and type of the random access tobe transmitted in order to allocate the random access data into the oneor more random access slots within the divided random access resource.21. A device for transmitting and receiving data in a code divisionmultiple access telecommunication system having random access resourcescontaining a plurality of random access slots for transmitting randomaccess data, said device comprising: means for dividing the randomaccess resources by assigning a signature which is available as one of16 separate codes on the basis of content and type of the random accessdata; and means for randomly choosing one or more random access slotsfrom the divided random access resource corresponding to the content andtype of the random access to be transmitted in order to allocate therandom access data into the one or more random access slots within thedivided random access resource.
 22. Method for transmitting andreceiving data in a code division multiple access telecommunicationsystem, comprising the steps of providing a random access channelcontaining a plurality of random access slots for transmitting randomaccess data, whereby each random access slot is defined by a time offsetvalue and a preamble signature, and allocating the plurality of randomaccess slots to different priority classes on the basis of the timeoffset values and preamble signatures, the priority classes representingtransmission priorities of the random access data to be transmitted inthe random access slots.
 23. A device for transmitting and receivingdata in a code division multiple access telecommunication system,comprising; means for providing a random access channel containing aplurality of random access slots for transmitting random access data,wherein each random access slot is defined by a time offset value and apreamble signature, and means for allocating the plurality of randomaccess slots to different priority classes on the basis of the timeoffset values and preamble signatures, the priority classes representingtransmission priorities of the random access data to be transmitted inthe random access slots.
 24. Method for transmitting and receiving datain a code division multiple access telecommunication system in which acommunication area is divided into cells, comprising the steps ofproviding a random access channel containing a plurality of randomaccess slots for transmitting random access data, whereby each randomaccess slot is defined by a preamble signature which is one of aplurality of signature codes differentiating the cells from each other,and allocating the plurality of random access slots to differentpriority classes on the basis of the preamble signatures, the priorityclasses representing transmission priorities of the random access datato be transmitted in the random access slots.
 25. A device fortransmitting and receiving data in a code division multiple accesstelecommunication system in which a communication area is divided intocells, comprising: means for providing a random access channelcontaining a plurality of random access slots for transmitting randomaccess data, whereby each random access slot is defined by a preamblesignature which is one of a plurality of signature codes differentiatingthe cells from each other, and means for allocating the plurality ofrandom access slots to different priority classes on the basis of thepreamble signatures, the priority classes representing transmissionpriorities of the random access data to be transmitted in the randomaccess slots.
 26. Method for transmitting and receiving data in a codedivision multiple access telecommunication system, comprising the stepsof providing a random access channel containing a plurality of randomaccess slots for transmitting random access data, whereby each randomaccess slot is defined by a preamble signature, broadcasting preamblesignature information to a communication device of the telecommunicationsystem, and randomly choosing at said communication device a randomaccess slot on the basis of the received preamble signature informationand transmitting random access data in the chosen random access slot.27. Apparatus for transmitting and receiving data in a code divisionmultiple access telecommunication system having a random access channelcontaining a plurality of random access slots for transmitting randomaccess data, whereby each random access slot is defined by a preamblesignature, said apparatus comprising: means for broadcasting preamblesignature information to a communication device of the telecommunicationsystem, and said communication device including means for randomlychoosing a random access slot on the basis of the received preamblesignature information and means for transmitting random access data inthe chosen random access slot.
 28. Method for transmitting and receivingdata in a code division multiple access telecommunication system,comprising the steps of: providing a random access time windowcomprising a plurality of random access slots for transmitting randomaccess data from at least one first communication device to a secondcommunication device, dividing the plurality of random access slots ofthe random access time window into at least two groups each having arespective initial size, and allocating the groups to respectivepriority classes, whereby the priority classes represent thetransmission priorities of the random access data to be transmitted inthe random access slots, whereby the size of at least one of said groupscan be changed in accordance with changing needs such that a probabilityof access for the at least one group can be changed dynamically; andwhereby contents of one of said groups are permitted to overlap withanother of said groups.
 29. Device for transmitting and receiving datain a code division multiple access telecommunication system, in which arandom access time window comprising a plurality of random access slotsfor transmitting random access data is provided, the plurality of randomaccess slots of the random access time window being divided into atleast two groups each having a respective initial size, and the groupsbeing allocated to respective priority classes, whereby the priorityclasses represent the transmission priorities of the random access datato be transmitted in the random access slots, means for randomlychoosing one or more random access slots from a group having a certainpriority class corresponding to the transmission priority of the randomaccess data to be transmitted, means for transmitting the random accessdata in said chosen one or more random access slot, whereby the size ofat least one of said groups can be changed in accordance with changingneeds such that a probability of access for the at least one group canbe changed dynamically; and whereby contents of one of said groups arepermitted to overlap with another of said groups.